Semiconductor image sensors are operated to sense light. Typically, the semiconductor image sensors include complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors, which are widely used in various applications such as digital still camera (DSC), mobile phone camera, digital video (DV) and digital video recorder (DVR) applications. These semiconductor image sensors utilize an array of image sensor elements, each image sensor element including a photodiode and other elements, to absorb light and convert the sensed light into digital data or electrical signals.
Front side illuminated (FSI) CMOS image sensors and back side illuminated (BSI) CMOS image sensors are two types of CMOS image sensors. The FSI CMOS image sensors are operable to detect light projected from their front side while the BSI CMOS image sensors are operable to detect light projected from their backside. The BSI CMOS image sensors can shorten optical paths and increase fill factors to improve light sensitivity per unit area and quantum efficiency, and can reduce cross talk and photo response non-uniformity. Hence, the image quality of the CMOS image sensors can be significantly improved. Furthermore, the BSI CMOS image sensors have high chief ray angles, which allow shorter lens heights to be implemented, so that thinner camera modules are achieved. Accordingly, the BSI CMOS image sensor technology is becoming a mainstream technology.
However, while existing BSI CMOS image sensors have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.